Method of anchoring nitrocellulose surface coatings and anchoring composition therefor



United States Patent 3 044,897 METHOD OF ANCHbRING NITROCELLULOSE SURFACE COATINGS AND AN CHORING COM- POSITION THEREFOR Vincent Dominic Celentano, Hamden, and Lamont Hagan, Guilford, Conn., assignors to Olin Mathieson Chemical Corporation, a corporation of Virginia No Drawing. Filed Jan. 30, 1959, Ser. No. 790,033

Claims. (Cl. 117,-73)

The subject invention relates to a method of anchoring a surface coating to a base surface, to compositions useful in the method and to the product formed. More particularly, the present invention relates to an improved anchoring agent for use in anchoring surface coatings to transparent, flexible sheets, to the methods of preparing and using these compositions, and to the articles formed.

A simplified flow diagram of the novel method is as follows Regeneratedcellulose sheet I Immerse in aqueous solution of softener and small amount of a partially polymerized anionic melamine-formaldehyde resin modified with benzoguanamine sulionic acid Dry impregnated film to moisture content of about 7% Coat with nitrocellulose lacquer composition containing 1.5 to

7% of an acid selected from the group consisting of itacomc,

acetic, acom'tic, hydroxyacetic and maleic acid Heat coated sheet to 115 0. for up to 90 seconds In recent years, transparent sheet wrapping material comprising a base sheet of softened, regenerated cellulose having a thin coating of a material containing nitrocellulose as a principal ingredient on its surface has be come an important article of commerce. Such coatings may contain a variety of ingredients to impart special properties such as resistance to moisture, low permeability to water vapor and the like to the finished article. The manufacture of a typical non-fibrous base sheet is described in Patent No. 1,548,864 (Brandenberger). Coating of this base sheet with typical coating compositions is described in Patent Nos. 1,737,187, 1,826,697, and 2,147,180.

A problem which has existed in the use of transparent wrapping materials formed with a regenerated cellulose base is the tendency of a thermal seal, formed by placing two layers of the material together and heating under slight pressure, to become separated due to ambient moisture conditions. This problem has existed for packages which are conventionally wrapped in a transparent regenerated cellulose base wrapping material, by folding and overlapping the edges in a conventional manner and heating the overlapping portions under slight pressure to seal the wrapping about the package. Under high humidity conditions, the seal formed between porv tions of the wrapping has become loosened due to the effect of moisture on the cellophane and the wrappinghas become separated from the article enclosed. a Inasmuch as the seal bond is formed largely between the coatings on the contacted surfaces of the sheets, the failure of the coatings to adhere to the base sheets can result in the same loss of Wrap from a package as a failure of the bond formed by the seal itself.

One way of reducing the separation of such surface coatings from the base sheets as well as the separation of the heat seals formed between such coatings is by in cluding in the base sheet an anchoring or bonding agent to bind the subsequently applied coating to the base surface. A number of problems have been found to result from the introduction of anchoring agents into the base sheet. 'llhe anchoring agent is usually incorporated into the base sheet by incorporating it into the softener bath. This softener bath may be, for example a bath of glycerol, propylene glycol, and the like. This bath is usually the last through which the base sheet passes prior to being dried in preparation for surface coating.

The treatment of the sheet prior to the softener treatment is accomplished as indicated in the above referenced patents by passing the sheet through various baths and washing solutions.

There is a tendency, due to the movement of the base sheet through the various baths, to transfer certain ingredients from these baths progressively from one bath to another. As a result, there is frequently found in a. softener bath a relatively high concentration of sulfate ion which has been carried into the bath by the base sheet.

One of the principal difficulties involved in imparting an anchoring agent to the base sheet as it is passed through the softener bath is that the agent tends to react With sulfate ion. As a consequence, the agent in the bath isrendered ineffective for its intended purpose and, in some cases, the bath itself must be replaced in its entirety to re-establish the favorable anchoring conditions. Accordingly, one object of the subject invention is to provide an agent for use in the softener bath which does not react with the contents of the bath. Another object is to provide an improved method of anchoring the surface coatings on base sheets. A further object is to provide a softener bath composition which is not subject to precipitation or other degradative reaction and which accordingly has an extended storage life. Still another object is to provide transparent wrapping articles prepared in accordance with the procedure taught herein.

Other object will be in part apparent and in part pointed out in the description which follows.

In one of its broader aspects, the objects of the invention may be achieved by providing, as a novel composition of matter, a water dispersion of melamine formaldethe use of this compound and the compositions in which it is used. The resin is preferably applied to a regenerated cellulose base sheet by incorporating it into a suitable softener bath to the extent of between 0.05 and 5.0% by weight. Sheets of regenerated cellulose are passed through this softener bath so as to permit sufficient softener and modified resin to be imparted to the sheet and so as to prepare them for retention of applied surface coatings. Prior to the surface coating, a sheet is dried to a water content of the order of 7% by passing it into contact with heated rolls.

The foregoing as well as the novel aspects of the subject invention will be more fully understood by consideration of the following example. This example is given for illustrative purposes and should not be interpreted as defining or limiting the scope of the invention.

EXAMPLE I To 54.7 grams (0.43 mole) of melamine in a one liter resin flask was added 154 grams (1.90 moles) of 37% formaldehyde in water. The pH of the formaldehyde had been adjusted to 8 by the addition of saturated solution of Na PO The mixture was heated for 25 minutes at a temperature of from 58 to 63 C. and, during this heating period, the melamine dissolved. A paste, formed from 23.2 grams (0.087 mole) of benzoguanaminesulfonic acid and 34.8 grams of a 10% solution of sodium hydroxide in water, was added to the solution with stirring. The pH of the solution was adjusted to 9 with a solution of sodium hydroxide in water.

The solution was heated for one hour at a temperature between 65 and 75 C., after which only a few particles of the paste remained undissolved. To this solution were added 195 grams (2.40 moles) of 37% formaldehyde in water and the solution was then cooled to a temperature below 40 C.

The solution was then rendered acid by adjustment of the pH to 5.0 by addition of a solution of 50% acetic acid in water. The acid solution was heated at a temperature of between 55 and 63 C. for five minutes.

The resulting resin solution was soluble in a solution of 5% Arquad 12 durin the warm-up period and after the first two minutes of heating at a temperature of 55 C. and above. At the end of three mintues, the addition of a small quantity of the solution to the 5% Arquad 12 solution resulted in the formation of a fine precipitate. After five minutes a similar addition resulted in the formation of a heavy precipitate. The resin solution was immediately cooled and 100 grams of absolute methanol was added with vigorous stirring. The pH of the solution was then adjusted to 7.0 by the addition of a 5% solution of sodium hydroxide in water.

The resin solution (630 grams) was then filtered and was observed to be a clear light tan liquid. This solution was soluble both in hot and in cold Water.

From the foregoing, it is evident that the total quantity of formaldehyde which had been added duringthe process was 4.3 moles, Thus, the ratio of formaldehyde to melamine used in the process was 10 to 1. The analysis of the resulting solution brought out that it contained 29.3 solids. In order to test the charge of the resin particles, 50 milliliters of a 5% resin solution was titrated with 5% Duponol. No precipitate formed with additions of up to 25 milliliters of Duponol. A white precipitate formed however with the addition of the first drop of 5% Arquad 12. A copious precipitate developed on further additions of the Arquad 12 solution up to about 6 milliliters of surfactant. The resin was accordingly found to be anionic.

In order to evaluate the resin solution as to its anchorage-inducing properties, the following baths were prepared.

1 Dodecyl trimethyl ammonium chloride. 2 Sodium lauryl sulfate.

Table I A (Control) B O 870 g. H20. 870 g. H30. 870 g. 13:0. 130 g. Solteuen 130 g. Softener. 130 g. Softener.

25 g. Resin. g. Resin.

Component: Weight percent Nitrocellulose 40 to Plasticizer 30 to 40 Blending resin 3 to 15 Wax l to 5 Polymeric plasticizer 5 to 15 The lacquer contained 15.5% solids in addition to 2.5% maleic acid based on the solid content of the lacquer. The lacquer was then dried for 30 seconds at a temperature of 112:2" C,

The samples were immersed in Water maintained at a temperature of C. and the times required for an indication of separation of the coatings from the base sheets were determined. The control sample A separated after two minutes of immersion. Samples B and C remained immersed in the water for more than 450 minutes without indication of failure of the coating.

Bath C was heated at a temperature of C. for a period of 5 to 10 minutes and was then heated for 7 /2 hours at a temperature of 72i2 C. At the end of this period, a small amount of precipitate was observed. This demonstrated the good stability of the bath and also indicated good keeping quality for long-term use.

It was also found that there were no significant concentration effects which could be related to the concentration of the resin in the bath. This finding was based on preparation of 8 samples, the first of which was a control and the remainder of which were samples of softener bath containing the indicated percentages and quantities of resin in the bath. Table II lists the sample number, the percentage of resin in the bath, the resin solids weight content of the bath, and the actual quantity of resin in each of the baths prepared. In each case, the bath contained 13% ethylene glycol. Samples of regenerated cellulose web were immersed in each of the baths for a period of 10 seconds and were then dried at a temperature of 114 C. for a period of seconds. The temperature of the baths was 75 C.

Table II t 4 1 Minutes Resin In Resln Resin In For Scpara- Sample Bath, Solids In Bath tion 01 Percent Bath (gins) (gms) Coating From Base Sheet 0 0 0 2 0.005 0. 05 0. 171 300 0. 010 0. l0 0. 341 300 0. 015 0. 15 0. 512 300 0. 10 1.00 3. l1 300 0. 15 l. 50 4. l2 300 0. 20 2. 00 6. 83 300 0. 30 3. 00 10. 24 300 Each of the samples was coated with the lacquer com position given above containing 15.5% solids and 2.5% maleic acid, and were then dried at 112 C. for 30 seconds. The anchorage values in water at 75 C. are given in the last column of Table II. From the table, it is evident that even at 0.005% resin solids in the bath, the anchorage was greater than 300 minutes.

The stability of the resin formed as above to the sulfate content of a softener bath is indicated by the data given in the accompanying Table III. In carrying out the tests of the eifect of the resin concentration of 2 baths, A" and B, was 0.5%. The baths A and B were formed by adding 130 grams of ethylene glycol, 870 grams of distilled water, and 17.1 grams of the resin, prepared as described above, to each of two, one liter stainless steel containers. On the accompanying Table III, the time, pH ofthe bath, temperature, quantity of sodium sulfate additive, and appearances of the bath are tabulated for the 8 hours test period.

' Table III pH Temp., G. M1. 1% Nags 04 Appearance B A B 1 pH raised from 5 to 6 with 5% N 2.011 to each. 2 Bath 11" had very slight foam. 3 Bath A had slight foam.

It was found that both baths remained perfectly clear throughout the test period.

EXAMPLE II A roll of regenerated cellulose sheet, containing about 250 feet of base sheet, was coated continuously at a rate of about feet per minute at a temperature of 112 to 115 C. in a lacquer coating apparatus. The lacquer used had a solids content substantially the same as that given above, containing about 12.5% solids and 2.5% maleic acid. The major portion of the coated film had a clear appearance, showing that the resin content of the regenerated cellulose base sheet used, did not affect the clarity 'of the coated film.

A second, or control, sample of regenerated cellulose film which was prepared did not contain the resin additive. The first sample coated as described above did contain the resin additive. The control sample was factory for this method is one which melts above 280 C. Examples of these acids are 2-sulfonic acid-4,6-diamino-s-triazine, 2-methanesulfonic acid 4,6 diamino-striazine and higher homologs of this compound such as Z-ethanesulfonic acid-4,6-diamino-s-triazine and these compounds may also be substituted for the benzoguanaminesulfonic acid.

The resin agent has particular utility in anchoring nitrocellulose coating onto regenerated cellulose base. However, it will be understood that other coatings such as coatings of cellulose acetate, butyrate and other esters, vinyl chloride copolymers, and vinylidene chloride copolymers may also be anchored by incorporating the resin agent in the base sheet and incorporating a few percent of an acid, such as maleic or aconitic acid, in the coating composition.

Such coatings may also be anchored .to the other base sheets, as for example to cellulose acetate base sheets or to other regenerated, non-fibrous pellicles.

Whereas the foregoing example is given with particular reference to maleic acid as the catalyst in the lacquer coating compositions, other acids such as itaconic, aconitic, hydroxyacetic, acetic, and others, may be substituted in a proportion of 1.5 to 7% of the lacquer composition without departing from the scope of the invention.

Where it is desirable to employ a base sheet, anchored as described herein, with a self-anchoring lacquer such as the lacquers disclosed in the co-pending applications for patent, Serial Numbers 757,935 and 757,936, filed August 29, 1958, they may be employed in anchoring a coating as they are compatible.

The preferred molar ratios used for forming the compositions disclosed herein are between 6 and 14 for the, ratio of formaldehyde to melamine used in the formation of the melamine formaldehyde resin, and between 7 and 2 for the ratio of the melamine to benzoguanaminesulfonic acid used in the formation of the modified resin.

Although the foregoing is described With reference to the use of a melamine formaldehyde resin, mixtures of urea and melamine may be used as well as mixtures of these resin monomers with dimethyl hydantoin or polyrnerized ethylenimine. The order of the addition of the reaction ingredients may be changed. Parafor-maldehyde coated by passing it through the same lacquer composition and then drying it. i V

A third sample of a regenerated cellulose base sheet, containing the same amount of anchoring resin as well as softener, was passed through a lacquer bath containing 12.5% lacquer solids and 3.5% maleic acid.

The samples were tested for their anchorage properties, or ability to retain the coating under moist conditions. The control film failed after immersion in Water at 75 C. for two minutes. Samples of the other two films had not failed after immersion in water at 75 C. for 40 minutes.

From the foregoing, it is apparent that the subject method provides a unique and novel method for improving the anchorage of surface coatings on base sheets of the regenerated cellulose type and the coating composition has unique stability with regard to freedom from percipitation of bath ingredients, as well as good resistance to reaction with sulfate ion. While the foregoing is described with particular reference to the melamine formaldehyde resin which has been modified with benzoguanaminesulfonic acid, it Will be understood that other sulfonic acid derivitives, such as those of diamino-s-triazines, may be substituted for benzoguanaminesulfonic acid without departing from the scope of the subject invention. Also, carboxylic acid derivatives, such as those of benzoguanamine, may be employed. A benzoguanaminesulfonic acid which has been found satisand other formaldehyde yielding materials may replace formaldehyde. Aldehydes other than formaldehyde or mixtures of formaldehyde with other aldehydes, such as ethylaldehyde or propylaldehyde, may be used. The reaction of these aldehydes is referred to herein as alkylolation. Softeners other than glycerol, such as ethylene glycol, 1,4-butanediol, diethylene glycol, triethylene gly-- col, and others, may be used to plastioize the base sheet.. The compositions useful in imparting good anchorage properties to base'sheets are preferably prepared by a procedure having four essential steps. 7

The first of these steps is the preparation of a soluble resin monomer by alkylolation of the monomer with.

form-aldehyde or compositions containing available formaldehyde. The degree of alkylolation which is preferred in carrying out this step is at least sufiicient to solubilize the resin monomer in an aqueous aldehyde alkylolating' agent. a moderate degree beyond that necessary for complete.

alkylolation of the replaceable hydrogen of the monomer or monomer mixture employed. It is advisable to avoid the presence of too great an excess of reacted aldehyde at this point in order to avoid reactions of this aldehyde with other agents, particularly modifier agents added in the subsequent steps. a

The degree of alkylolation should be limited to amass? 7 pH values, below 7.5, may also be employed, but the risk of condensation polymerization increases as the pH is lowered. A good operating temperature range is between 55 and 90 C., and the time of reaction may be from 10 to minutes, however, operation outside of these indicated temperature and time ranges is feasible.

A second essential step of this reaction is the treatment of the resin monomer, which has been alltylolated to form a polyalkylol resin monomer, with an amino acid modifying agent under alkaline conditions. The amino acid useful in carryin out the subject process must contain at least two amino groups containing a sum of at least three potentially replaceable hydrogens. It has been discovered that increased anchorage can be attained through the use of the agents of the subject invention by employing modifier reactants which have a high empirical hydrogen content, where at least three such hydrogens are attached to nitrogen atoms and are reactive with aldehyde and alkylol groups.

It is in this second step that a control of the ratio of formaldehyde to resin monomer must be made with great care. The ratio of both reacted and unreacted aldehydes to the available hydrogens attached to the monomer nitrogens must not appreciably exceed one, as in such case reaction can occur between the free formaldehyde and the modifier added by the second step. As noted above, the lower ratio of formaldehyde preferred for use in this process is that which will result in substantially complete solubilization of the resin monomer.

A preferred pH range is between 7.5 and 10, although pH values up to 12 may produce satisfactory results. At pH values below 7.5, the tendency toward condensation polymerization increases. Temperatures betwen 65 and 90 C. are preferred, although the process is operable outside of this range. A reaction time, for this reaction temperature range, of between and 90 minutes is satisfactory. In general, the adequate time of reaction can be determined by observing the condition of thereaction mixture. This is so because when the modifier is first added, a suspension forms. As the reaction proceeds, this suspension goes into solution. Thus, the clearing of the solution indicates the completion of the reaction. Stirring maintains the reactants in a preferred condition for early completion of the reaction.

At this point, i.e., after the reaction is substantially complete, a considerable amount of formaldehyde may be added without any deleterious effects.

The third essential step of the process is the polymerization of the modified resin monomer. To effect such polymerization, the pH of the reaction mixture is reduced. This may be accomplished by the addition of mineral or organic acids soluble in the reaction mixture. Such addition of acid should follow a cooling of the reaction, preferably below 40 C., in order to avoid a run-away condensation polymerization reaction. lows the polymerization inducing acid agent to be added to, and homogeneously mixed with the reaction product of step 2 prior to the commencement of an appreciable rate of polymerization.

The polymerization rate and extent is then controlled by controlling the temperature of the mixture. The quantity of acid added prior to this heating determines the rate of time required for completion of the polymerization at any given temperature.

The fourth essential step of the process is the termination of the polymerization reaction at a point where the product is partially polymerized. This may be carried out by various conventional techniques.

One such technique is by diluting and neutralizing of the reaction mixture. A second is by cooling and neutralizing the mixture. A third is by adding alcohol or polyhydric alcohol agents. This latter procedure has been found to improve the storage stability of the product formed.

The presence of a hazy colloidal composition having This cooling alsome slight coloration indicates the formation of the desired product.

A method for determining the attainment of the desired end point for the reaction is illustratively described in the example. The procedure involves the periodic addition, durin the third step, of a drop of the reacting mixture into a surfactant having a known charge which is opposite to that desired in the product of the subject method. The behavior of the drop in this surfactant is observed. An end point is indicated by the first formation of a heavy fibrous precipitate as a test drop is added. The attainment of the desired end point in this reference surfactant means the attainment of the desired degree of polymerization of the modified resin product.

While it is not understood what mechanism accounts for the superior results obtained in the practice of the subject invention, a common property of all of the modified anchoring agents employed in accordance herewith is that they have two activated amino groups. During the curing period and opportunity for cross-linking, greater than those provided by other agents, may accordingly be afforded.

Since many examples of the foregoing procedures and articles may be carried out and made, and since many modifications can be made in the procedures and articles described without departing from the scope of the subject invention, the foregoing is to be interpreted as illustrative only, and not as defining or limiting the scope of the invention.

We claim:

1. The partially polymerized reaction product of a melamine resin monomer which has been rendered water soluble by alkylolation of the replaceable hydrogen of said resin and an amino acid modifying agent, said acid containing at least two amino groups containing a sum of at least three potentially replaceable hydrogens.

2. The partially polymerized product of claim 1 wherein the modifying agent is benzoguanamine sulfonic acid.

3. The partially polymerized product of claim 2 wherein the molar ratio of alkylolated melamine to benzoguanaminesulfonic acid is between 7 and 2.

4. As a composition of matter, an anionic water dispersible partially polymerized reaction product of melamine monomer which has been rendered Water soluble by alkylolation with formaldehyde, with benzoguanaminesulfonic acid modifying agent.

5. The composition of claim 4 in which the molar ratio of alkylolated melamine to benzoguanarninesulfonic acid is between 7 and 2.

6. An anionic anchoring resin for improving the anchorage of surface coatings to base surfaces whichcomprises the partially polymerized anionic reaction product of melamine monomer alkylolated to render it water soluble with an amino acid containing at least two amino groups and at least three replaceable hydrogens attached to nitrogen.

7. The resin of claim 6 wherein the acid is an animo sulfonic acid.

8. A method of preparing a water dispersible anionic melamine formaldehyde resin modified With benzoguanaminesulfonic acid which comprises reacting formaldehyde and melamine in a molar ratio of between 6 and 14, reacting the melamine formaldehyde formed thereby with benzoguanaminesulfonic acid in a molar ratio of between 2 and 7 and thereafter partially condensing the reaction product.

9. The method of claim 8 in which the condensation is allowed to proceed until a drop of the reaction mixture forms a precipitate with a cation-active surface active agent and terminating the reaction by cooling and neutralizing to a pH of 7 to 8 with a hydroxide solution.

10. The method of anchoring nitrocellulose Surface coatings to regenerated cellulose base sheet which comprises softening the base sheet with an aqueous composition containing about 15% softener and a fractional percentage of a partially polymerized anionic melamine formaldehyde resin modified with benzoguanaminesulfonic acid, drying this base sheet to a water content of about 7 thereafter coating said sheet with a lacquer composition containing dissolved nitrocellulose and from 1.5 to 7% of an acid selected from the group consisting of itaconic, acetic, aconitic, hydroxyacetic, and maleic acid, heating the coated sheet to a temperature of about 115 C. for a period up to 90 seconds.

References Cited in the file of this patent UNITED STATES PATENTS Thurston Sept. 25, 1945 Thurston Dec. 4, 1945 Ellis Sept. 26, 1950 Lindenfelser et a1. Sept. 4, 1956 Brown Oct. 11, 1960 

10.THE METHOD OF ANCHORING NITROCELLULOSE SURFACE COATINGS TO REGENERATED CELLULOSE BASE SHEET WHICH COMPRISES SOFTENING THE BASE SHEET WITH AN AQUEOUS COMPOSITION CONTAINING ABOUT 15% SOFTENER AND A FRACTIONAL PERCENTAGE OF PARTIALLY POLYMERIZED ANIONIC MELAMINE FORMALDEHYDE RESIN MODIFIED WITH BENZOGUANAMINESULFONIC ACID, DRYING THIS BASE SHEET TO A WATER CONTENT OF ABOUT 7%, THEREAFTER COATING SAID SHEET WITH A LACQUER COMPOSITION CONTAINING DISSOLVED NITROCELLULOSE AND FROM 1.5 TO 7% OF AN ACID SELECTED FROM THE GROUP CONSITING OF ITACONIC, ACETIC, ACONITIC, HYDROXYACETIC, AND MALEIC ACID, HEATING THE COATED SHEET TO A TEMPERATURE OF ABOUT 115* C, FOR A PERIOD UP TO 90 SECONDS. 